1. Field of the Invention
This invention relates to a device for transferring hot glass sheets from a shaping station to a cooling station, and particularly relates to the type of transfer device that returns empty to the shaping station for transferring a succeeding hot glass sheet to the cooling station to perform a mass production method for shaping and heat strengthening or tempering glass sheets.
2. Technology Problems and Patents of Interest
The prior art has provided devices for transferring hot glass sheets which comprise an outline or ring-type mold of relatively rigid metal that includes an outline upper surface shaped to conform to the shape desired slightly inward of the outline of the glass sheet. The glass sheet is heated and either pressed to shape between complementary molds and transferred to the ring-type mold or sagged by heat and gravity to the shape provided by the upper shaping surface of the mold. When the glass sheet is shaped, it is transferred while supported on the ring-type mold to a quenching station where it is quenched suddenly and rapidly by applying cold air blasts against the supported lower surface and against the upper exposed surface of the glass sheet.
Heating the glass sheet to its softening temperature followed by sudden chilling develops a stress pattern characteristic of tempered glass that includes a surface zone highly stressed in compression surrounding an interior zone stressed in tension. This stress pattern reduces the tendency of the tempered glass sheet to break, particularly if the surface zone is highly stressed. Furthermore, if tempered glass is fractured, it forms a number of small, smoothly surfaced particles that are less dangerous than relatively large, jagged fragments that result from the breakage of untempered glass.
When a shaped glass sheet contacts a hot, heavy metal rail during this rapid cooling, glass breakage can result from surface vents due to high tension stress established locally at the glass-metal contact areas. The prior art has recognized that the difference in heat conductivity between the glass sheet supported on the mold and the relatively heavy mold ring during rapid cooling causes these high tension stresses in the hot shaped glass sheet in the areas contacting the glass supporting rail of heavy metal. Hence, in recent years, the art has developed a ring-like member having a glass sheet supporting surface for the ring-type mold. The ring-like member is composed on non-metallic material having a low coefficient of thermal conductivity. The most effective solution to date uses a ring-like member having a glass engaging surface consisting essentially of a phenolic resin, preferably polyphenyl formaldehyde, reinforced with fibers composed of an aromatic polyamide composition. This preferred reinforced material is popularly termed aramid and will be so identified hereafter in this specification.
The material used to engage the glass is reinforced either by thickening its lower portion, which makes a ring-like member used as a non-metallic tempering ring awkward to handle because of the thickness needed for rigidity, or the glass engaging material is hugged and surrounded by a relatively heavy metal rail slightly larger than the outline of the supported glass sheet to impart rigidity to a ring-like member composed of a preferred non-metallic glass engaging material. Tempering ring structures comprising a ring-like member of aramid reinforced with a hugging heavy metal support ring have produced results superior to the prior art by maintaining the glass sheet in contact with the fiber reinforced phenolic resin material in spaced relation over the heavy metal rail of the tempering ring. However, still better results are desired to be consistent with an objective of manufacturing larger glass sheets of more complicated shapes than in the prior art with even less marking or venting than previously experienced.
U.S. Pat. No. 3,973,943 to Seymour discloses an outline ring-like transfer device for supporting shaped glass sheets after they are shaped and during the time they are rapidly quenched. The device includes a rigid, outline metal rail having an outline slightly smaller than that of the shaped glass sheet and a bar or plurality of closely spaced bar members of non-metallic material having a lower heat transfer coefficient than that of the rigid metal rail mounted in hugging relation against the rigid metal rail in position to conform to the outline rail. An upper edge portion of the bar or bar members is disposed above the upper edge of the rigid metal rail in a position to provide a glass sheet supporting surface in spaced relation above the upper edge of the rigid metal rail. The bar or bar members of the Seymour patent are composed of a laminated structure, such as fiber glass cloth or asbestos woven paper layers or the like, bonded together with a silicone resin binder or a phenolic resin binder. The glass engaging materials of this patent have been replaced by superior materials disclosed in the following patent.
U.S. Pat. No. 4,525,196 to Fecik et al. discloses a structure in which an exterior metal rail slightly larger than the outline of the supported sheet is used to reinforce a bar or bar portions of phenolic resin reinforced with aramid fibers to provide a support plane inboard of the metal reinforcement rail and above the upper edge of the metal reinforcement rail. The superior physical characteristics of the phenol resin (preferably polyphenyl formaldehyde reinforced by aramid aromatic polyamide fibers) is discussed in this patent and its disclosure, including a detailed discussion of these characteristics, is incorporated within this specification by reference. It is noted in passing that the glass engaging member spaces the glass, particularly its edge portion, from the heavy metal reinforcement rail but does not shield the glass from direct exposure to radiation from a closely adjacent heavy metal reinforcing rail.
U.S. Pat. Nos. 4,556,407 and 4,556,408 to Fecik et al. disclose spaced blocks supported by spaced metal rails above the level of the metal rails to provide spaced support members of a non-metallic material that supports hot glass sheets for tempering. The glass engaging members are adjustable in position relative to the spaced rails in both patents and may be pivotal, as in U.S. Pat. No. 4,556,407. The glass engaging members of these patents do not shield the glass from direct exposure to radiation from closely adjacent metal rails.
U.S. Pat. Nos. 4,282,026 and 4,361,432 to McMaster et al. disclose the use of a helical spring wound over a solid heavy metal tempering ring closely adjacent to the glass to space the glass from direct contact with the heavy metal tempering ring and thus reduce the cooling rate differences between the glass and the metal. The helical spring in these patents do not shield the glass from direct exposure to radiation from a closely adjacent tempering ring.
In addition, U.S. Pat. Nos. 4,363,163 and 4,421,482 to McMaster disclose non-metallic materials such as aromatic polyamid fibers wound around metal rotating conveyor rolls. However, these latter patents are limited to the use of such materials as a covering for rotating conveyor rolls for conveying hot glass sheets during thermal treatment and do not suggest using these materials to shield glass sheets from direct exposure to radiation from closely adjacent heavy metal tempering rings.